1. Field
One embodiment of the present invention relates to a disk drive. More particularly, the invention relates to a technique of generating a synchronous clock for the write operation in a disk drive.
2. Description of the Related Art
In most disk drives, a representative example of which is a hard disk drive, the head performs a write operation to record data on a disk that is a magnetic recording medium, and performs a read operation to reproduce the data from the disk.
A write clock synchronous with the rotational speed of the disk (hereinafter referred to as a synchronous clock) is used in order to write data in the write operation performed by the write head that is included in the head. In normal operation, a spindle motor rotates the disk at a constant speed.
Two types of write operations are performed in the disk drive. One type is performed to write user data to the disk. The other type is performed to write servo data to the disk during the manufacture of the disk drive. The servo data is data that is used to control the positioning of the head over the disk, and is recorded in the servo sectors provided on the disk. In either type of write operation, a sync-clock generator that generates a synchronous clock must be used.
The sync-clock generator must be feedback-controlled to adjust the clock frequency and the like, in order to generate a clock that is synchronous with the rotational speed of the disk. Hitherto, the clock frequency has been adjusted by some methods. In one method, the time interval between any two adjacent servo sectors is measured by using servo-sync marks recorded in these servo sectors, and then the frequency of the synchronous clock is adjusted in accordance with the time intervals thus measured. (See, for example, U.S. Pat. No. 7,088,534.) In another method, the time interval between the trigger patterns recorded in the disk is measured, and then the phase of the synchronous clock is adjusted in accordance with the time intervals thus measured. (See, for example, U.S. Pat. No. 6,324,027.)
Both conventional methods specified above can indeed generate a clock that is synchronous with the rotational speed of the disk. In these methods, however, no controls based on the controllability of the spindle motor that rotates the disk are performed at all. Consequently, the rotational speed of the disk greatly changes the moment the spindle motor is accelerated and decelerated. Inevitably, the synchronous clock cannot be adjusted, faithfully in accordance with the rotational speed of the disk changes.